Various types of photochromic compounds are known for use in light transmitting articles such as lenses, filters, screens, and windows. These photochromic compounds change color on exposure to certain wave lengths of light or other electromagnetic radiation. These photochromic changes are reversible, that is, they change to and from the various colors on exposure to and withdrawal from the activating light radiation. It is known that the general basis of the photochromic reaction is produced by a photochromic substance which contains atoms or molecules capable of switching back and forth between or existing in two distinct energy states. The substances are induced into a higher energy state by absorption of the activating radiant energy which is generally of specific wave lengths defined by the particular materials, usually in the ultraviolet, and, in the absence of the activating radiant energy, return to their inactivated stable states. In the color or activated states, they absorb certain ranges of light energy, and in the inactivated state, the important photochromic materials pass most wave lengths of electromagnetic energy in the visible portion of the spectrum.
Silver halide particles have been found to be a very useful photochromic material and glass has been the preferred matrix for photochromic silver halide particles. The silver halide particles are typically formed in situ in molten glass using high temperature techniques. After the glass has been formed and appropriately annealed to generate photochromic particles, ultraviolet and short wavelength visible light causes the silver halide particles to decompose to elemental silver and halogen atoms Glasses are believed to provide a microscopic environment wherein the halogen atoms remain in close proximity to the elemental silver for recombination after removal of the activating light. In addition, the halogen atoms do not appear to participate in irreversible reactions with other components of the glass so that the halogen remains available for the reverse reaction.
The size and shape of the halide particles may also be important in commercial photochromic products, especially transparent products. If the particles are much bigger than about 1,000 .ANG., Rayleigh scattering of light occurs to produce a hazy lens that is visually unacceptable.
Where it is intended to incorporate silver halides into a polymeric matrix, the silver halide particles need to be shielded from the chemical effects of the matrix that may have a deactivating effect on the photosensitive particles. The deactivating effect is believed to result in part from the easy oxidation of the silver halide by peroxide initiators, catalysts, and even the polymer matrix itself. Although a number of interesting techniques have been tried in an attempt to duplicate, in photochromic polymers, the performance of silver halide particles in a glass matrix, such attempts apparently have apparently met with little success.
PCT International Patent Application WO 89/12839 to Wasserman, et al. discloses the formation of photochromic polymer membranes that are used on light transmissive materials such as lenses, window glazings, car windshields, camera filters, and the like to control UV and visible radiation and glare.
A silver halide emulsion is formed from solutions of silver ions and halide ions to give silver halide particles that are smaller than about 800 .ANG. using established methods for the preparation of silver halide emulsions for photographic use. A protective environment for controlled silver halide growth is provided by adding a water soluble polymer that does not bind silver or halide ions irreversibly (polyvinyl pyrrolidone, polyvinyl alcohol, polycarboxylic acids, polysulfonic acids, polyethers and co-polymers thereof).
The protective water soluble polymer may be added to the initial halide or silver ion solutions or both or it may be added to the solution of silver halide particles after they have formed. If an initial protective polymer material is used, it is removed during the washing of the silver halide particles. Photoactivating agents such as copper(II), copper(I) or a combination thereof together with sulfur-bearing ions such as FS.sup.-, S.sub.2 O.sub.3.sup.=, or combinations thereof with R being an organic radical are added to the emulsion in a concentration of 10.sup.2 to 10.sup.5 parts per million (based on the silver content of the emulsion) to serve as photoactivating agents.
The initial protective polymer is replaced with a higher molecular weight water-soluble polymer that also does not irreversibly bind halogens. The resulting emulsion of surface-activated silver halide in suspension with a suitable polymer can be coated onto suitable substrates such as glass or polymeric light transmissive materials. It is asserted that the polymer must be one that loosely binds halide ion and that a polymer containing at least 50% halogenated groups enhances the reversibility of the silver halide.
U.S. Pat. No. 4,049,567 to Chu, et al. discloses the preparation of polymer matrices, with particular emphasis on polyvinyl pyrolidone and polyvinyl alcohol, containing activated silver halide particles of less than 1,000 .ANG.. Lithium, sodium, potassium, rubidium, cesium, thallium, copper(II), calcium, magnesium, strontium, barium, zinc and beryllium ions can be substituted for up to 50% of the silver ions. Copper(I), iron(II), cadmium and sulfide ions are used as activating ions. The abovementioned polymers act as crystal growth inhibitors. The product requires a plasticizer such as water, glycerine, ethylene glycol, polyethylene glycol, and mixtures thereof to produce an environment suitable for repetitive activation and deactivation of the silver halide particles. It is stated that the polymer must retain the plasticizer to keep its photochromic properties and may be sealed between glass plates to prevent the loss of the plasticizer and hence photochromic activity.
U.S. Pat. No. 4,556,605 to Mogami, et al. discloses a photochromic coating composition for synthetic resin ophthalmic lenses. The coating composition includes an organic silicon compound or its hydrolyzate and silver halide as a photochromic material dispersed therein. A number of Japanese patent applications disclosure similar type organo-silicon materials, for example, JP 59-214002 A, JP 60-125802 A, JP 60-136702 A, and JP 61-93401 A.
U.S. Pat. No. 4,110,244 to Hovey discloses the preparation of a silver halide in transparent polyester materials by first forming a transparent polymer, swelling a surface of the cured polymer with a polar solvent such as methanol, absorbing silver and halide ions into the swelled surface layer and evaporating the solvent to cause the swelled surface layer to collapse, trapping silver halide particles in the surface layer of the plastic.
U.S. Pat. No. 4,046,586 to Uhlmann, et al. discloses the preparation of silver halide in polymer compositions for ophthalmic use. Particles of silver halide, with dimensions between 30 and 10,000 .ANG. and internally doped with Cu.sup.+ or other cations are first formed. A coating of a halogen impervious layer of metal oxide such as Al.sub.2 O.sub.3, SiO.sub.2, or TiO.sub.2 is formed around the particles allegedly to prevent diffusion of halogen out of the crystal and to render the crystal sufficiently resistent to the plastic host material or components of the host material such as the monomers and peroxides that are used in the polymerization of the host materials.
U.S. Pat. No. 4,106,861 to Brewer, et al. discloses a photochromic protective layer system alleged to be characterized by a reduced haze. Light transmissive articles are formed by evaporating photochromic silver halide onto a plastic coated with a material substantially impermeable to halogens. The silver halide is coated with a layer of metal, such as gold, platinum, palladium or chromium and then laminated to another sheet of plastic coated with a material substantially impermeable to halogens.
U.S. Pat. No. 4,578,305 to Postle, et al. discloses a photochromic assembly in which crushed photochromic glass beads are imbedded in a polymer latex. Other examples of the use of photochromic glass particles include West German patent application DE 3308186 A1, UK patent application GB 2,144,433 A, Japanese patent applications JP 60-262155 A and JP 63-64019 A.
While the photosensitivity of silver halide particles has been found useful in photographic imagings, only an irreversible photo-induced chemical change is sought in silver halide containing photographic materials. Recombination of the elemental silver and halogen in photographic film would lead to destruction of the latent photographic image. Examples of photographic materials are provided in U.S. Pat. No. 4,431,730 to Urabe, et al. and U.S Pat. No. 4,591,328. U.S. Pat. No. 4,713,322 to Bryan et al., describes a photographic silver halide emulsion that contains thioether compounds that improve silver halide crystal growth.
U.S. Pat. No. 4,913,845 to Gillberg-Laforce, et al., illustrates metallic silver colloids dispersed in a composite polymer matrix which materials exhibit a nonlinear optic response.
U.S. Pat. No. 4,714,692 to Abrevaya, et al., discloses a reverse micelle technique for preparing a microemulsion impregnated catalyst composite. Steigerwald, et al., J. Am. Chem. Soc., 1988, 110, 3046-3050 discloses the synthesis of CdSe using an inverse micellar solution with chemical modification of the surface of the cluster compounds by covalent attachment of organic ligends. U.S. Pat. No. 4,122,030 to Smith, et al., pertains to the formation of colloidal dispersions of selenium by a locus control method. U.S. Pat. No. 4,701,218 to Barker, et al., deals with a size stabilizer for ultra fine silver particles (0.5-3,000 nm so that the silver particles can be dispersed for in various materials for use as transparent coloring agents.
U.S. Pat. No. 3,875,321 to Gliemeroth, et al. deals with a fused glassy or crystalline AgBr-CuBr system for use as a reversible phototropic coating applied to glass and plastic substrates.
U.S. Pat. Nos. 4,596,673 and 4,687,679 to Beale disclose a monomer that is codeposited with silver halide particles of 25-150 .ANG. size onto transparent glass or plastic by plasma polymerization or glow discharge polymerization. PCT international application WO 85/00432 to Merle discloses a silver halide photochromic film prepared from a silica or phosphate base material and a silver halide that is subsequently deposited on a plastic article such as a lens by using a vacuum deposition technique. U.S. Pat. No. 4,018,807 to Brooks, Jr. pertains to the synthesis of aromatic sulfur-oxygen transition metal complexes, such as silver or copper, from aromatic hydroxy mercaptans. Some of the complexes such as the nickel and cobalt complexes can be incorporated into plastics for the manufacture of optical products that protect against laser radiation. U.S. Pat. No. 3,806,462 to Bloom relates to plastic optical elements that include a layer having an organometallic complex that is an infrared absorber. The organometallic complexes are formed from transition metal elements such as copper and an aromatic or heterocyclic ring that binds to the metal through oxygen and sulfur linkages. The infrared absorbing complex can be imbibed into transparent plastic sheets. U.S. Pat. No. 3,576,755 to Patella, et al. is drawn to photochromic compositions of an oxygen containing polymer, such as a polycarbonate, and a transition metal complex of various metals, but not including silver or copper.
As apparent from this brief summary, glass has been the most reliable host matrix for photochromic silver halide. However, the major drawbacks to the use of glass as a matrix for photochromic substances are its weight and the high cost of manufacture. Attempts have been made to impart silver halide-based photochromic properties to windows, ophthalmic lenses and other articles made from transparent polymeric materials that are lighter and less expensive to manufacture than glass and that mimic the properties of photochromic glass. However, such attempts have not been particularly successful and typically involve the use of water-soluble polymer matrix material. Other more costly processes involve the use of silicon hydrolyzate materials, molecular organometallic complexes, vacuum deposition or glow discharge polymerization.